414 V. E. EMMEL 



4. Certain sti-uctural modifications and frequent discontinuity 

 in the endothelium at the base of these masses, the cytological 

 characteristics transitional between these endothelial cells and 

 the component cells of clusters, the evidence of mitotic activity, 

 variation in size, and relationship to certain degenerating aortic 

 rami, support the conclusion that the aortic clusters have arisen 

 from the vascular endothelium. 



5. An intimate association and fundamental causal relation- 

 ship can be demonstrated between the formation of the aortic 

 clusters and the developmental processes involving the atrophy 

 of certain aortic rami and the establishment of the pemianent 

 intestinal arteries of the adult mammal. The endothelium 

 in degenerating stems of the aortic rami is stimulated, (evidently 

 through certain toxic conditions arising in the retrogressive 

 vessels), to phagocytic and proliferative activities giving rise 

 to infra-arterial cell masses constituting a primary source of 

 origin of the aortic clusters. 



6. On the basis of the cumulative evidence of various recent 

 investigators it appears evident that the original assumption 

 of the angioblast theory that the endothelium of the general 

 systemic vascular system does not contribute to the cellular ele- 

 ments of the blood, while possibly true under normal conditions, 

 requires the qualification that under certain abnormal conditions 

 endothelial tissue ordinarily passive may in both embryo and 

 adult assume such proliferative activities. ^ 



5 While the present paper was in press an article appeared in the Anatomical 

 Record, Vol. 10, p. 417, by Jordan on the "Evidence of Hemogenic Capacity 

 of Endothelium." It is of especial interest to note that Jordan records the 

 observation of cellular structures in the aorta of mongoose and turtle embryos 

 apparently similar to the clusters occurring in the pig, mouse and rabbit em- 

 bryos. Here again the clusters are confined to the ventral region of the aorta. 

 In the mongoose and turtle, just as in the pig, "the clusters show a progressive 

 increase in size corresponding with the age of the embryos, between 5 and 10 

 mm., indicating an intrinsic growth" p. 419. It is emphasized that "similar 

 clusters are found nowhere else either in the j'olk sac or the embryonic vessels 

 or sinusoids" (p. 418) and in agreement with the results of the present paper 

 cogent reasons are advanced for regarding these clusters as being not mc:ely 

 chance cellular accumulations, but as structures arising from the vascular en- 

 dothelium. 



